The present invention relates to coatings on cemented carbide bodies, and particularly, to the use of boride coatings on cemented carbide substrates to obtain firm bonding and improve wear resistance.
Cemented carbides are well known for their unique combination of hardness, strength, and wear resistance, and are accordingly extensively used for such industrial applications as cutting tools, drawing dies and wear parts. For abrasive wear and nonferrous metal-cutting applications, WC-Co compositions are preferred because of their high strength and good abrasion resistance. For steel machining applications, compositions consisting of WC-TiC-TaC-Co, TiC-Ni or TiC-Ni-Mo are preferred because they are less reactive with steel workpieces at high machining speeds. The use of carbides other than WC generally results in a significant strength reduction, however, which limits either the amount of TiC and other carbides that can be added or the severity of the application when large amounts of TiC are used.
The use of carbide, nitride, and carbonitride coatings on cemented carbide is well known as a way to improve wear resistance in machining and metal turning applications. However, it has been found that such coatings do not possess the requisite hardness for purely abrasive wear situations such as are encountered in the drilling of rock and coal cutting. In addition, the carbide, nitride, and carbonitride coated articles cannot be brazed to steel holders because the liquid braze metal will not wet the carbide, nitride, and carbonitride coatings.
The use of boride coatings represents an improvement over carbide, nitride and carbonitride coatings. More particularly, boride coatings, such as TiB.sub.2, are harder than carbide and nitride coatings, such as TiC and TiN, and thus, more useful in purely abrasive wear situations such as coal cutting and rock drilling. In addition, boride coatings are easily wet by braze metals, and thus, the boride coated articles can be brazed to steel holders. This greatly facilitates their use in articles such as coal cutters and roof support drilling tools which must be brazed.